When driving around, your van is producing massive amounts of electricity from the alternator which can be used to charge your leisure batteries.
A B2B charger is the most effective tool for this job because it creates a multi-stage charge for the leisure batteries from the simple system voltage provided by the alternator.
B2B Charger vs Battery Isolator
Battery isolators essentially pair the leisure batteries with your existing vehicle electrical system so that the alternator thinks it’s just charging a bigger starter battery. This is a simple way to grab power and functions fine at a relatively low cost.
Where isolators falls short is they don’t provide a full multi-stage charge to your leisure battery.
Because of system resistances and the voltage that the starter battery is charged, isolators will charge your system relatively slowly when filling the leisure batteries the last bit of the way up. This is because an alternator is designed to keep a starter battery topped off, not recover it from deep discharge. It provides charge based on the system resistance when running which slows down as the batteries get more full. Once the leisure batteries are charged up to 80% or so, the alternator output will be at a trickle charge- sometimes less than 5A.
A B2B charger resolves these issues. A B2B charger uses the same energy from the alternator – but more of it and in a more efficient way.
B2B chargers all work essentially the same: They connect to the starter battery and pull as much power from it as they safely can. This makes the alternator work harder to keep up with the power draw, similar to turning on a bunch of lights and stereo in your car would. The B2B charger then takes this power and steps up the voltage – which remember is coming int at about 13.8V – and increases it to appropriate fast absorption rates for your leisure batteries. The real power and efficiency of a B2B charger comes from this step in the process.
All good B2B chargers will make sure that your starter battery doesn’t get drained faster than the alternator is charging it. This is done by limiting the voltage drop – usually to around 13.1V to make sure the starter battery stays topped off.
If you can afford the up-front cost and are relying on electricity to function on the road, a B2B charger is one of the first pieces you should upgrade from a standard isolator in your system.
Which B2B Charger Should You Get:
The US market has pretty limited options for B2B chargers. There are only two readily accessible companies with a few different combinations of products to consider. Fortunately, both are very reputable and there are options for all system sizes.
These all work in a similar way. They are attached directly to your starter battery. They don’t turn on until the system voltage reaches charging levels (so they won’t drain your starter when the car is off). This ensures that the alternator is still charging the starter battery but maximizes the amount of energy that the B2B gets from the alternator.
|Sterling BB1230||Sterling BB1260||Sterling BBW12120||CTEK D250SA||CTEK D250SA+Smartpass|
|Input Charge Rating||30A||60A||120A||20A||140A|
|Built-in Solar MPPT||YES, up to 20A, 12V (~450W)||YES, up to 20A, 12V (~450W)|
|Lead Acid Battery Size||300Ah||300-600Ah||400-800Ah||100-200Ah||800Ah|
|Water Resistance||Low- IP21||Low- IP21||Very High- IP68||High- IP65||High- IP65|
|Battery Temp Sensor||✓||✓||✓||✓||✓|
|Price||Check Price||Check Price||Check Price||Check Price||Check Price|
Things To Look For In A B2B Charger
Input Charge Rating
This is the max amount of charge the system can take from your starter battery. The output will vary slightly based on the voltages being set for multi stage processing. But it is fine to connect a 30A charger to a system with a 120A alternator- it won’t cause any problems.
The CTEK unit comes with a built in solar MPPT controller. This controller is rated at a 20A input, which can’t work with much more than 450W of panels. If you go this route, you must have a 12V input and check that the panel VOC isn’t above the max rated 23V for the CTEK unit. For the Sterling units, you’d install an MPPT separately.
Largest Lead-acid Size
This is our recommendation based on average charging capabilities of AGM batteries. You can charge larger batteries with many of them, but there are potential sulfation issues with slow charging. It’s fine to go outside of this range as long as you know the limitations.
Lithium batteries require custom programmable charge specifications that many units cannot do. For units that can create custom lithium charge profiles, we still recommend batteries with a built in BMS to minimize potential charging issues.
Battery Temp Sensor
Chargers can monitor battery temp can reduce current or completely shut off to protect batteries from being damaged.
Wrapping It Up
B2B chargers are unique because they can do a full multi-stage charge on your batteries, which is something that an alternator can’t do through a standard isolator. Most vandwellers are fine with the functionality and simplicity of an isolator, but in the long run a B2B is more efficient and better for your battery health.